The invention relates to a gas analyser comprising a Wheatstone bridge, consisting of resistance wires, which are provided in two mutually separated channels of a measuring block, wherein the resistance wires of opposite legs of the bridge are contained in the same channel, while a reference gas may be conducted through a first one of said channels and a gas to be analysed may be conducted through a second one of said channels.
The operation of such a gas analyser is based on the thermal conductivity properties of gases. The bridge is adjusted in such a manner that the temperature of the resistance wires lies above the room temperature, the bridge being balanced when the same gas mixture is conducted through both channels. When, for example, air is used as reference gas and the gas to be analysed is a mixture of air and one other gas, the output voltage of the bridge varies in dependence on the concentration of said other gas because this gas conducts heat more or less than air.
A problem of the conventional gas analysers of the type described is that the output voltage of the bridge is non-linearly dependent on the concentration of the gas to be analysed. This is caused by the fact that the thermal conductivity of gas increases with increasing temperature, whereby for gases conducting heat better than air, as for example helium, each next unit increment of concentration contributes less to the output voltage than the preceding unit increment, whereas for gases conducting heat less than air, as for example carbon dioxide, each next unit increment of concentration contributes more to the output voltage than the preceding unit increment. Therefore, the conventional gas analyser can only be used for measuring low concentrations of gas, for example up to a maximum concentration of 5%.
it has been proposed to provide a greater measuring range by applying the output voltage of the bridge, after being amplified to a suitable value, to a diode resistance circuit to linearize the output voltage. This means that the gauging characteristic of said circuit should be well reproduceable because otherwise an economic production is not possible. Further, the diode resistance circuit should always be provided with the same input voltage for a certain gas concentration. Moreover, such a linearization is only valid for one gas.